1. Field of the Invention
The present invention generally relates to a sampling method, and more particularly, to a sampling inspection method.
2. Description of Related Art
Typically, a semiconductor production line may be carried out with a plurality of process steps, e.g., film preparing step, microlithography process step, etc. Besides the process steps, the semiconductor production line is usually carried out with a plurality of inspection steps for inspecting products of each of the process steps.
In each inspection step, a process performance of a product lot with respect to a process step is usually deducted as either acceptable or defective by inspecting physical characteristics of every product or semi-product of the production lot. Currently, most inspections performed on production lines are sampling inspections. In a typical sampling inspection procedure, different sampling inspection ratios are set for different machine types regarding different process steps. Products of different lots are then randomly sampled in accordance with the sampling inspection ratios for inspection. When there are only a few machines performing a same process step in a production line, such a sampling inspection method performed to different production lots in accordance with the sampling inspection ratios is feasible, because each of the machines has an approximate probability to be sampled for inspection within a short period when performing each of the process steps. However, when the production line is in a large scale, and there are many machines employed in the production line, the sampling inspection method performed to different production lots in accordance with the sampling inspection ratios may result blind spots which may be sampled for inspection after too long a time.
In such a way, when a processing failure occurs in a machine, which causes defective products in a same lot of products, while unfortunately it happened that the process performance of the machine has not yet been sampled for inspection, the production yield will be affected, and the reliability of the inspection will be impaired. However, it is almost impossible to one by one inspect process performance of each of the multiple machines of the large scale production line, because it inevitably increases the complexity of inspection procedure and consumes too much time on the inspection procedure.